Radio search tuning system



Oct. 10, 1950 A. F. BISCHOFF mm smnca 'ruumc svsmu 2 Sheets-Sheet 1 Filed Dec. 31, 1947 Eizwhom .m

EMT/Bowl Inventor- Alfw-ed F. Bischoff.

by 7%.0M

His Attorney.

Oct. 10, 1950 A. F. BISCHOFF RADIO SEARCH TUNING SYSTEM Filed Deb. $1, 1947 2 Sheets-Sheet 2 v m m W F H m m lmm- I iliiiw liilll1||l| |l+ e at 1 H 3 vfiflwzmw I e .l mm H u m W m wit/Gout 2.. age w +19 P N.

L WW mm 59:5 8. $02 mm 99? tm m m: we v m mh Patentedoct. 10, 1950 RADIO SEARCH TUNING SYSTEM Alfred F. Bischofl", Ballston Spa, N. Y., assignor to General Electric Company, a corporation of New York Application December 31, 1947, Serial No. 794,923

Claims. (Cl. 25040) This invention relates to automatic radio tuning systems and more particularly to a search tuning system for radio receivers by means of which the receiver is scanned through a band of frequencies and automatically tuned to a desired 1 signal.

It is an object of this invention to provide a tuning system for radio receivers wherein the receiver circuits are tuned through a band of radio signals and caused to seek out a signal above a predetermined adjustable level and whereby the receiver is accurately tuned to the frequency of this signal.

It is a further object of this invention to provide a tuning system for radio receivers of the push button type which obviates the necessity of pretuned circuits are the use of mechanical means for tuning these receivers.

Yet another object of this invention is to provide a tuning system for a radio receiver wherein the receiver circuits are automatically tuned through a band of frequencies until a signal above a predetermined adjustable threshold is intercepted and wherein electronic means, actuated by the intercepted signal, are provided which accurately tune the receiver circuits to the frequency of this signal.

The features of my invention which I believe to be new are set forth with particularity in the appended claims. together with further objects and advantages thereof, may best be understood by reference to the following description when taken in conjunction with the accompanying drawings, in which Fig. 1 shows one embodiment of my search tuning system and Fig. 2 shows a preferred modification thereof.

I Referring now to Fig. 1, this figure shows a conventional superheterodyne receiver adapted to receive amplitude modulation and frequency modulation signals and further adapted to be tuned by the search tuning system which is embodied in the present invention.

The radio frequency amplifier stage of the receiver is designated as and signals from this amplifier stage are heterodyned in a mixer stage 2 with oscillations from a local oscillator stage 3. The resulting intermediate frequency signals are amplified inone or more intermediate frequency amplifier stages 4. The output from the intermediate frequency amplifier stage 4 is applied to a conventional frequency discriminator 5. The discriminator comprises the usual frequency discriminator transformer 6, rectifying devices 1 and 8 and load resistors. 9 an My invention itself, however,

l0. Amplitude modulation signals may be taken across the load resistor H] and frequency modulation signals may be taken across the differentially connected load resistors 9 and H1, network acting as a filter to remove any intermediate frequency components from the audio output. The amplitude modulation and frequency modulation outputs of the discriminator 5 are applied to the contacts 2 and I3 respectively of a switch I4 and in this manner amplitude modulation or frequency modulation signals may be supplied to an audio amplifier stage |5 through a capacitor l6 and thence to the usual translating device IT.

The output of the frequency discriminator 5 taken across the load resistors 9 and I0 is also applied to the control electrode I8 of a discharge device I9 through a resistor 20. The discharge device i9 is cathode coupled to a discharge device 2|, the cathodes 24 and 25 of these devices being connected to ground through a resistor 25. These devices form a direct current amplifier stage, and are coupled in the well known manner so that the potential developed in the output of device I9 is of opposite polarity to the potential developed in the output of device 2|. The control electrode 22 of the device 2| is connected to ground through a resistor 21, and to a resistance network 23 through a resistor 29 and an adjustable potentiometer 28 of network 23. The control electrode l8 of device I9 is connected to network 23 through a resistor 30. Unidirectional biasing potential is applied to the control electrodes of devices 9 and 2| from source B+ through a resistor 3| and resistance network 23, as shown. The above described circuit forms a balancing arrangement for devices l9 and 2|, and the potentiometer 28 is adjusted so that for zero output from discriminator 25 the anod potentials of devices 19 and 2| are of equal amplitude and opposite polarity.

The output of the direct current amplifier" stage is taken across load resistors 32 and 33 which are connected respectively to the anodes 34 and 35 of devices I9 and 2|. The potentials appearing across the load resistors 32 and 33 are applied respectively to the control electrodes 35 and 37 of two balanced control discharge devices 38 and 39 through resistors 40 and 4|. The cathodes 42 and 43 of the devices 38 and 39 are connected together and further connected to ground through a resistor 44. The anodes 45 and 46 of the devices 38 and 39 are connected to a center-tapped coil 41 of a reversible tuning a r gement 48.

The tuning arrangement 48 comprises a motor 49, having armature terminals I58 and H59, which drives a shaft 50, this shaft passing through the coil 47 and carrying a pair of disks 5i and 52. The shaft'50 and the disks 5! and 52 are constructed of soft iron or similar material and become magnetized in a direction depending upon the resultant magnetic field of the coil 47. The direction of this resultant field and hence the magnetism of the shaft 5!] and the disks 5! and 52 is dependent upon whether the anode current of the device 38 or the anode current of the device 39 predominates, and when these anode currents are equal the resultant magnetic field becomes zero. A further disk 53 of soft iron or similar material is connected by means of a swivel .ioint to a threaded shaft 56 in such a manner that rotation of the d sk 53 rotates the shaft 54 and thereby controls the 'receiver tuning mechanism, shown in this example as permeability .tuned coils 55 and 56. A permanent magnet 5'! is placed beneath the disk 53, the magnet 5'? having the form of a disk of substantially the same diameter as the disk 53, the magnetism of the ma net 5'11 be ng of greater strength than the ma netism of the shaft 5! and the disks 5| and 52. It can be seen therefore that for one direction of magnetism in the sha t 55 and the d sk 5! and 52 the disk 53 will take a positi n shown by the solid lin s and en a e one d sk 52. and hence by a fricton drive the s aft 54 will be rotated and the tuning coils 55 and 55 will be tuned in one direction. When the direction of the magnetism in the shaft '5'! and the disks 5| and 52 reverses, the disk 53 takes the po ition shown by the dot t d l nes and the direct on of rotation of the shaft 54 is reve sed. It is apparent t at there is no rotation of the sha t 51! when the resultant ma netic fie d of the coil 41 is zero.

In the above de crib d system a control circu t for a radio receiver is provided and when the receiver is tun d to a, signal any deviation in the receiver from the c nter f eo ncv of this si nal gives r se to a control potential acro s the load re i tors 9 and M of the fre encv di criminator 5, the si n and amnlitnde of this control otential being d endent u on th direction and magnitude of the deviation. Th control potent al is amp i ed in the balanced direct current amplifier and applied to the b lanced control devices 38 and as previou ly de cribed. Any unbalance in the out ut of the d vice 38 and'39 caused by the cont ol potential will drive the tuning mechanism 48 in such a direction as. to restore th balance and hence retune the receiver to the center frecuency of the received si nal.

The system thus far described mav be termed the automa ic frecuencv control. section of the search tuning system and the fol owing description is directed. to the means whereby the receiver is scanned throu h the fr quency band and automat cally tuned to a de ired si nal having a ma nitude above a predetermined thresho d.

The output of the di criminator 5 a earin across the load resi tor it is a li d to the control electrode 53 of a gaseous di charge device 59 through a re istor 58, the control electrode 58 being cou led to ground throu h a ca acitor 5| capacitor Bl acting as a bvpass for extraneous noise and other short dura ion im ulses. In the absence of a de ired signal. the device 59 is bia ed to a nonconductive state by means of a unidirectional potential from a source C- which is applied to the control electrode 5%! by way 0i a P0- tentiometer 62 and a resistor 63. The potentiometer 52 is variable and provides a threshold control for the device 59. The anode 64 of the device 59 is connected to the control electrode 65 of an electron discharge device 56 through a resistor 61 and to a contact 68 of a relay 69 through a resistor 70. The control electrode 55 of the device 66 is connected to a negative biasing source C through a resistor TI. The cathode 12 of this device is grounded and the anode 13 is connected to the anode 34 of the device IS. A contact 14 of the relay B9 is connected to a source of unidirectional potential B+ and when the relay 69 is energized the contacts 68 and 14 close and unidirectional potential is applied to the anode 54 of the device 59 through a resistor 15. Further contacts 15 and 16 of relay 69 also close when the relay is energized to connect motor 49 to a source of motor potential.

When device 59 is non-conducting, the volt- .age on anode 54 is of such a value that it overcomes the negative bias on the control electrode 55 of the device 55 and drives this control electrode slightly positive which causes a current flow in the device :66, thereby unbalancing the control devices 38 and 39 and hence causing the receiver tuning mechanism to tune the receiver through the frequency band. This tuning continues until a signal having an amplitude sumcient to overcome the adjustable threshold on the gaseous discharge device 59 is intercepted. When such a signal is received, device 59 becomes conductive and the potential of anode 54 falls to such a value that the bias on the control electrode of'device 65 returns this device to its nonconductive state and no longer exerts an unbal.

ance on control devices 38 and 39. The previously described automatic frequency control then becomes operative'accurately to control the receiving tuning mechanism for optimum reception of the incoming signal.

The relaylifl 'includesa coil 85 and one terminal of this coil is connected to a source of unidirectional potential 3+ and to contact 19 of a push button H. The other terminal of coil 85 is connected to the anode 85 of the discharge device 82, the cathode 81 of this device being connected to ground. The contact of pushbutton TI is connected to the control electrode 8! of device 82 through a resistor 83 electrode 8! being connected to a source of negative biasing potential C- through a resistor 8 which normally biases device 82 to a noncondu'ctive state. A capacitor F8 is connected between contact .85 and ground for the purpose to be described.

The relay 69 is energized by means of 3, m0- mentary depression of pushbutton ll. This m0- mentary depression applies potential from'the source 28+ to capacitor '58 charging this capacitor. The charge on capacitor 18 positively biases the control electrode 8! and device 82 becomes conductive, thus completing to ground the circuit of coil of the relay 59 and energizing this relay. Relay 69 remains energized until the charge on capacitor lit leaks ofi through resistors 83 and 84 and when the charge has leaked off, device 32 returns to its nonconductive state. The time constant of the capacitor 18 and resistors 83 and 85 is made such that relay 69 remains energized for a time sufficient to allow the tuning circuit to select a signal and accurately tune the receiver to this signal.

The pushbutton H is provided with two normally closed contacts 88 and 89. These contacts ar i c ded n the cathode circuit of the gaseous discharge device 59. Therefore, although the device 59 may be conducting due to a-received signal before pushbutton TI is depressed, the depression of the pushbutton energizes the relay 85 and in the manner previously described tunes the receiver away from this received signal and the depression of the pushbutton also opens the cathode circuit of the device 59 which assures that the device 50 is completely deionized and prepared for the next signal as the pushbutton I1 returns to its normal position.

The tuning mechanism 48 includes a return switch 90 and when the mechanism is at the top of its travel an arm 9| moves a switch arm 92, closing a contact 93 and thus applying unidirectional potential from the source 33-}- through the control electrode 36 of the discharge device 38 through a resistor 94. This unbalances the control devices 38 and 39 and the tuning mechanism is returned to the bottom of its travel, the contact 93 remaining closed until the arm 9i coacts with the sw tch arm 95 to reopen this contact.

From the above description it is apparent that when pushbutton I1 is depressed for a moment and then released, relay 69 is energized for a predetermined period and motor 44 shifts the tuning mechanism away from the station presently being received. The gaseous discharge device 59 is deionized due to the fact that when the pushbutton I1 is depressed, the cathode circuit of this device is opened and when the pushbutton is returned to its normal position the motor 49 has shifted the tuning mechanism away from the station formerly received and the device 59 is in its non-conductive state. The receiver scans the frequency band, as previously described, until a signal of strength sufficient to overcome the bias on the device 59 is received and this restores the balance to the control devices 38 and 39, and the automatic frequency control tunes the receiver accurately to the received signals.

When the receiver has been'tuned to the received signal, the charge on the capacitor I8 reaches a point where the device 82 again becomes nonconducting and the relay 69 becomes deenergized, and the search tuning circuit is switches to its standby state, the circuit remaining in this condition until the pushbutton I1 is again depressed and the above described sequence of events again initiated. When. the search tuning circuit is in its standby state, the motor 49 is deenergized and anode potential is removed from the devices I9, 2|, 59, 66, 38 and 39 and hence the system consumes substantially no power except when it is in actual operation.

Referring now to the modification shown in Fig. .2, wherein like numerals indicate elements similar to those shown in Fig. 1, I have again shown a conventional superheterodyne radio receiver comprising a radio frequency amplifier stage I, a mixer stage 2, a local oscillator stage 3 and an intermediate frequency amplifier stage 4. The intermediate frequency amplifier stage 'I is coupled to a frequency discriminator 96 of the type utilizing a single multi-grid electron discharge device. The output of the intermediate frequency amplifier 4 is applied to the frequency discriminator 96 by way of a coupling circuit which includes an inductance 91 and a capacitor 98. The coupling circuit also includes a rectifying device 99, as shown, and amplitude modulation audio signals may be taken across the resistor I00 which is bypassed for intermediate frequencies by a capacitor l0I. These audio signals may be applied to the contact I2 of a switch similar to switch I4 of Fig. 1 and thence to the audio output stages of the receiver.

Th discriminator 96 comprises a discriminator transformer I02 having a primary winding I03 tuned by a capacitor I04 and a secondary winding I05 tuned by a capacitor I06. One side of the primary winding I03 is connected to the control electrode I01 of the electron discharge device I08 through a' resistor I09, and one side of the secondary winding I05 is connected to a signal electrode I I0 of the device I08 through a resistor III, the signal electrode H0 being shielded by the screening electrodes I I2 which are connected to a source of unidirectional potential B+ through a resistor H3 and bypassed by a capacitor H4. The cathode II5 of device I08 includes the usual cathode biasing resistor I I6 which is bypassed by a capacitor II! and the cathode is also positively biased from the unidirectional source B+ through a resistor H8. The anode H0 of device I08 is connected to the unidirectional potential source B+ through a resistor I20 and bypassed to ground by a capacitor I2I. The discriminator transformer I02 is so tuned that when an intermediate frequency signal of a frequency lower than the frequency to which the discriminator transformer I02 is tuned, is received, the anode potential of device I08 increases above its normal value, and conversely, when an intermediate frequency signal higher in frequency than that to which the discriminator transformer I02 is tuned, is received, the anode potential of device I08 decreases below its normal value. Therefore, an audio output potential appears at the junction of anode I I9 and resistor I20 in response to frequency modulation in the intermediate frequency signal. This audio output may be applied to contact I3 of a switch similar to switch M of Fig. 1. The resistors I09 and I I I are included in the control and screened electrode circuits as in their absence when an intermediate frequency signal reaches such a magnitude that these'electrodes are driven positively, the discharge device H9 acts as a limiter and the audio signals appearing across the resistor I00 are distorted.

It is pointed out that as the tuning control of the receiver is advanced from the low frequency side of a signal to the high frequency side, the anode potential of device I 03 first rises to a maximum peak and then falls to a minimum peak, passing through the normal potential between the peaks and returning again to the normal potential after, the signal is completely passed. In

the present search tuning system the receiver is tuned from the low frequency side to the high frequency side of a desired signal and the rise of anode potential of the device I08 to a maximum peak and the fall of this potential to the normal value at the center'frequency of the signal is used to control the tuning of the receiver.

The output of frequency discriminator 95 is applied to'the control electrode I22 of an electron discharge device I23 through a resistor I24 and a resistor I 25, the control electrode being cou led to ground through a capacitor I40. The cathode I31 of the device I23 is positively biased bya potential derived across the potentiometer resistors I38 and I39 connected between the unidirectional source 13+ and ground. The resistor I39 is variable and acts as a threshold control for the device I23. A neon tube MI is included in the cathode circuit of the device I23 and acts as a source of reference potential and 7 as'anaindicator. tube,. as this tube willjglow more brightly when a signal is. approached and the device I23 draws current. The anode I26 of'the device I23: is connected to. the unidirectional source B+ through a resistor I2! and a relay coil I28. The amount of current through thecoil I28 may be adjusted by means of a variable resistor I42, as shown. One terminal of a relay coil I35 is connected. to the. junction of the resistor I2! and the coil: I28 through series resistors I29 and I36. The other-terminal of the coil I35 is connected to ground. One terminal of a pushbutton I30 is connected to the. junction point of the resistors I29 and I36, and the other terminal of thispushbutton is connected to a contact I34 of the relay I28 and to a contact I3I of the relay I35. Further contacts I43 and I44 of the relay I23 are connected to a resistor I33, as shown, the contact I43 being grounded and the contact I44 being additionally connected to one terminal of a reversing switch I32. Contact I46 ,of relay I35 is connected to one terminal I of a' source of motor potential. The other terminal I41. of the motor source potential is connected to ground.

Contact I46 is also connected to terminal I63 on limit switch I50. The common switching contact I45 of relay I35 is connected to terminals I6I', I82 of reversing switch I32 and contact I45 is also connected to terminal IE4 on limit switch I50. The third contact I of reversing switch I32 is connected to common switching contact I44 of relay I28. The common switching contacts I66, I6I of reversing switch I32 are connected to supply energizing potential to the armature terminals I 63, I69 of motor 49. The reversing switch I32 is connected to the tuning motor 49 which drives the tuning mechanism 48 through a gear box I48 and the threaded shaft 54. The tuning mechanism includes a switching device I49 which reverses the switch I32 at one end of the travel of the tuning mechanism 48,

- thus returning the tuning mechanism to the other end of its travel where the switching device I43 again reverses the switch I32.

When the receiver is switched on, current flows from the unidirectional source B+ through the relay coil I28, the resistor I29, the pushbutton I30, the contacts I3I and I45, the reversing switch I32, the motor 49 and the resistor I33 to ground, this current being sufficient to energize the relay coil I28' but insufficient to turn the motor 49. The relay coil I 28 is therefore energized and the contacts I 44 and 934 are closed which forms a holding circuit for the relay I28, through the resistor I29, the pushbutton I30, the closed contacts I34 and I44 and the resistor I33 to' ground. Some current passes to the relay coil I35 through the resistors I29 and I36 but this current is insufficient to actuate the relay I35.

When the pushbutton I30 is depressed the holding circuit of the relay I28 is broken and this relay becomes deenergized, and the resistor I33 is short-circuited by the now closed contacts I43 and I44. Current is diverted to the relay I35 through the resistors I29 and I 36, the variable resistor I42 being adjusted-to such a value that the relay I 26 is not actuated under this condition. The relay I35 is therefore energized and closes the contacts I45 and I46 energizing the motor 49, and the tuning mechanism tunes across the frequency band. When a signal is approached, as previously described, there is a rise of anode potential of the device I08. If this potential riseis sufficient to overcome the threshold estab- 8 lished by the resistor I39, the device I23"becom'es conductive. The relay coils I28 and I35 are each included in the circuit of the device I23 and when this device conducts, coil I28 becomes energized, closing the contacts I44 and I34, thus inserting the resistor I33 in the circuit of the tuning motor 49 and the motor operates at a reduced speed, the coil I35 remaining in its already energized state. As previously described,

1 the control potential applied to the control electrode I22 of the device I23 rises to a maximum peak and thence returns to normal as the receiver is tuned from the low frequency side of the signal to the center frequency. 1 The relays I28 and I35 are therefore energized for this period and when the center frequency is reached and the control potential returns to normal, the device I23 returns to its non-conductive state and the relays I28 and I35 simultaneously become deenergized, the contacts I34 and I43, I45 and I3I close and a short circuit. is therefore'placed across the armature of the motor 43 which acts as a dynamic brake and'the motor promptly stops at the center frequency of the desired signal.

The neon tube I4I glows and when asignal is approached and the glow becomes brighter as the control potential rises to a maximum peak and gradually fades as the center frequency "of the signal is approached.

The tuning mechanism 48 is equipped with a switching device I49, as previously described, which causes the motor 49 to reverse when the tuning mechanism reaches the end of its travel. When the tuning mechanism 48 reaches the high frequency end of the frequency band the switching device I49 also closes a switch I 5I, connecting the control electrode I22 of the device I23 to ground and making this device unresponsive to received signals. The switch I50 is also closed at this point and the motor 49 is directly energized and the tuning mechanism is returned to the low frequency end of the frequency band where the switch I32 is again "reversed, switches I53 and I5I are opened and the receiver is once more conditioned to scan the frequency band and to be automatically tuned to a received signal. The control line I52 can also be connected to the usual audio muting means to eliminate unnecessary noise in the receiver as the tuning mechanism 48 is being returned to the low frequency end of its travel.

The search tuning system described above obviates the necessity of pretuned circuits or preset mechanical means to effect the tuning of a radio receiver. This system moreover causes the receiver circuits to scan the frequency band until a signal of desired strength is received and automatically tuned to this signal. 1

While certain specific embodiments have been shown and described, it will of course be understood that various modifications may be made without departing from the invention. The appended claims are therefore intended to cover .any such modifications within the true spirit quencies in a predetermined direction, means for deriving a control potential from said frequency discriminator in response to a signal intercepted by said receiver and means utilizing said control potential for disabling said unbalancing means thereby to arrest the motion of said tuning mechanism.

2. An automatic tuning control for a radio receiver comprising a frequency discriminator, a pair of balanced electron discharge control devices and a, tuning mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for unbalancing said control devices causing said controlled tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means for deriving a first control potential from said frequency discriminator in response to a signal intercepted by said receiver, means utilizing said first control potential for disabling said unbalancing means thereby to arrest the motion of said controlled tuning mechanism, means for derivin a second control potential from said frequency discriminator, and means for impressing said second control voltage on said control devices, thereby controlling the motion of said controlled tuning mechanism for accurately tunin said receiver to said intercepted signal.

3. In a radio receiver, the combination of a plurality of balanced electron discharge control devices, a movable tuning mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for unbalancing said control device thereby to cause said tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means including adjustable threshold means for deriving a control potential in response to a signal intercepted by said receiver when said intercepted signal has an amplitude greater than said adjustable threshold, and means utilizing said control potential to disable said unbalancing means thereb to arrest the motion of said tuning mechanism.

4. In a radio receiver, the combination of a frequency discriminator, a plurality of balanced electron discharge control devices, a movable tuning mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for unbalancing said control devices to cause said tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means responsive to a signal intercepted by said receiver for deriving a control potential, means utilizing said control potential to disable said unbalancing means, means for deriving an automatic frequency control potential from said discriminator in response to said intercepted signal, means for impressing said automatic frequency control potential on said control devices in a balanced state for accurately tuning the receiver to said intercepted signal.

5. In a radio receiver, an automatic tuning control comprising, movable tuning means for tunin said receiver through a band of frequencies, frequency discriminating means, means for deriving an automatic frequency control potential from said frequency discriminating means in response to signals intercepted by said receiver, control means for moving said movable tuning means, means for applying said automatic frequency control potential to said control means to cause said tuning means accurately to center on a first intercepted signal, means for applying a searching potential to said control means to cause said tuning means to move in a predetermined direction in search of a second signal within said frequency band, means for deriving a second control potential from said frequency discriminating means in response to a second signal intercepted by said receiver, and means responsive to said second control potential for disabling said searching potential apply ing means, whereby said automatic frequency control potential causes said tuning means ac curately to center on said second signal.

6. In a, radio receiver, an automatic tuning control comprising, movable tuning means for tuning said receiver through a band of frequencies, frequenc discriminating means, means for deriving an automatic frequency control potential from said frequency discriminating means in response to signals intercepted by said receiver, control means for moving said movable tuning means, means for applying said automatic frequency control potentialto said control means to cause said tuning means accurately to center on a first intercepted signal, means for applying a searching potential to said control means. to cause said tuning means to move in a predetermined direction in search of a second signal within said frequency band, means for deriving a second control potential from said frequency discriminating means in response to a second signal intercepted by said receiver, means responsive to said second control potential for disabling said searching potential applying means, whereby said automatic frequency control potential causes said' tuning means accurately to center on said second signal, and means for applying a'tnird control potential to said control means to cause said tuning means to traverse said frequency band in the opposite direction without interruption from signals intercepted by said receiver.

7. In a radio receiver, the combination of a frequency discriminator, a plurality of balanced electron discharge control devices, a movable tuning mechanism for the receiver, means controlled by said control devices for moving said tun1ng mechanism, means for connecting the output of said discriminator to said control devices thereby accurately to tune said receiver to a received signal, means for supplying an unbalancing search potential to said control devices to cause said tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means responsive to a signal intercepted by said receiver for developing from said frequency discriminator a control potential, and means including adjustable threshold means for disabling said search potential applying means when said control potential has an amplitude greater than said adjustable threshold, whereby said frequency discriminator causes said receiver accurately to tune to said intercepted signal.

8. In a radio receiver, the combination of a frequency discriminator, a plurality of balanced electron discharge control devices, a tuning mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for connecting the output of said discriminator to said control devices thereby accurately to tune said receiver to a received signal, means for unbalancing said control devicesto cause said tuning mechanism to tune the receiver through a band of frequencies in a prede termined'direction, means responsive to a signal intercepted by said receiver for developing from said frequency discriminator a, control potential, means including adjustable threshold means for disabling said unbalancing means when \said control potential has an amplitude greater than said adjustable threshold, whereby said discriminator causes said receiver accurately to tune to said intercepted signal, and means for unbalancin said control devices when the limit of said frequency band is reached to cause said tuning mechanism to traverse said frequency band in the oposite direction without interruption from signals intercepted by said receiver.

9. In a radio receiver of the type having a frequency discriminator, an automatic tuning control system comprising, a plurality of balanced electron discharge control devices, a tuning mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for connecting the output of said discriminator to said control devices thereby accuratel to tune said receiver to a received signal, means for supplying an unbalancing search potential to said control devices to cause said tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means responsive to a signal intercepted bysaid receiver for developing from said frequency discriminator a control potential, means including adjustable threshold means for disabling said search potential applying means when said control potential has an amplitude greater than said adjustable threshold, whereby said discriminator causes said re-.- ceiver accurately to tune to said intercepted signal, time delay means initiated by said search potential applying means, and means controlled by said time delay means for disabling said automatic tuning control system.

10. In a radio receiver of the type having a frequency discriminator, an automatic tuning control system comprising, a plurality of bal= anced electron discharge control devices, a tun-' ing mechanism for the receiver, means controlled by said control devices for moving said tuning mechanism, means for connecting the output of said discriminator to said control de vices thereby accurately to tune said receiver to a received signal, means for unbalancing said control devices to cause said tuning mechanism to tune the receiver through a band of frequencies in a predetermined direction, means responsive to a signal intercepted by said receiver for developing from said frequency discriminator a control potential, means including adjustable threshold means for disabling said unbalancing means when said control potential has anamplitude greater than said adjusta'blethreshold, whereby said discriminator causes said receiver accurately to tune to said intercepted signal, means for unbalancing said control devices when the limit of said frequency band is reached to cause said tuning mechanism to traverse said frequency band in the opposite direction Without interruption from signals intercepted b said receiver, time delay means initiated by said first mentioned unbalancing means, and means controlled by said time delay means for disabling said automatic tuning control system.

ALFRED F. BISCHOFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number, Name Date 2,056,200 Lowell Oct. 6, 1936 2,369,542 Dietrich Feb. 13, 1945 2,375,133 Polkinghorn May 1, 1945 

